Solar flares and solar mass ejections can induce a rise in ground potential in the areas where the charged particles interact with the earth's magnetic field. When electrical power transmission lines connect two points with substantial potential difference in the two ground potentials, a direct current (DC) is transmitted over the transmission line, in the same way that a DC current is transmitted over a wire connecting two battery terminals. Transmission lines are normally terminated to a large power transformer that transforms the transmission line voltage to a different level for generation or distribution purposes.
When significant levels of DC current are applied together with AC voltages and currents, the DC can offset the normally balanced flux in the transformer core and cause the core to saturate every half cycle. The degree of saturation is dependent on the design of the core, the electrical characteristics of the steel used in the core, and the level of DC current applied to the core. A saturated core will cause large spikes in the transformer currents which can cause severe noise and over-heating problems. Catastrophic transformer failures have occurred as a result of Geo-magnetic induced currents that the transformers were not protected against.
Different types of transformer designs are more susceptible to damage from Geo-magnetic induced current events. While the detection of DC current in the transformer's neutral is a primary indicator of a Geo-magnetic induced current event or other abnormal DC conditions, the level of even and odd harmonics are also good indicators of the degree of core saturation and resulting damage that can occur to each individual transformer. FIG. 3 shows 3 common types of transformer winding configurations 300. Autotransformers 302 are widely used for transmission line transformation, but Wye-delta transformers 304 or Delta-wye transformers 306 may be used in some applications. Transformers with neutral terminals on the transmission-side windings (typically the HV windings) are most susceptible to Geo-magnetic induced current effects, but other design criteria such as single-phase and/or shell-form transformer designs can also have a significant influence of the susceptibility to damage from Geo-magnetic induced current events.
There are established methods of protecting transformers from Geo-magnetic induced current events. Some options include the use of DC shunting or blocking filters or switches to remove the transformer from the transmission line, but all such protection systems depend on an accurate detection of the Geo-magnetic induced current condition which the various embodiments herein provides.
The measurement of DC neutral current is standard industry practice and utilized in all current art Geo-magnetic induced current monitoring systems, but the current art Geo-magnetic induced current monitoring system, utilizes auxiliary current transformers which are susceptible to saturation and inaccurate performance under high current or currents with a significant amount of DC current.
The current art also utilizes stand-alone bushing monitoring systems and stand-alone Geo-magnetic induced current monitoring systems. The embodiments herein are the first to combine the two functionalities into one common transformer monitoring device.